Only preserve PRIMARY and CLIPBOARD selection ownership by default

Only preserve PRIMARY and CLIPBOARD selection ownership by default

* lisp/cus-start.el (standard): Update defcustom type.
* src/xselect.c (x_should_preserve_selection): New function.
(x_clear_frame_selections): Use it to determine whether or not
to preserve a selection.
* src/xterm.c (x_preserve_selections): Fix tail initialization.
(syms_of_xterm): Update doc string of
`x-auto-preserve-selections'.